In the refining of lubricant base stocks, a series of generally subtractive processes is employed to remove undesirable components from the process feedstock. The most important of these processes include atmospheric and vacuum distillation, deasphalting, solvent extraction, and dewaxing. These processes are basically physical separation processes in the sense that if all the separated fractions were recombined it would reconstitute the crude oil.
Refineries do not manufacture a single lube base stock but rather process several distillate fractions and a vacuum residuum fraction. Generally, at least three distillate fractions differing in boiling range and the residuum may be refined. These four fractions have acquired various names in the refining art, the most volatile distillate fraction often being referred to as the "light neutral" fraction or oil. The other distillates are called "intermediate neutral" and "heavy neutral" oils. The vacuum residuum, after deasphalting, solvent extraction and dewaxing, is commonly referred to as "bright stock." Thus, the manufacture of lubricant base stocks involves a process for producing a slate of base stocks, which slate includes at least one refined distillate and one bright stock. Additionally, each subtractive step produces a byproduct which may be processed further or sold to an industry which has developed a use for the byproduct.
Conventional processing of crude petroleum oil to recover fractions suitable for upgrading in various refinery processing operations employs multi-stage distillation. Crude oil is first distilled or fractionated in an atmospheric distillation tower, with residual material from the bottom of the distillation tower being further separated in a vacuum distillation tower. In this combination operation, gas and gasoline generally are recovered as overhead products of the atmospheric distillation tower, heavy naphtha, kerosene and gas oils are taken off as distillate side streams and the residual material is recovered from the bottom of the tower as reduced crude. Steam may be introduced to the bottom of the tower and various side strippers used to remove light material from withdrawn heavier liquid products. The residual bottoms fraction or reduced crude is usually charged to a vacuum distillation tower. The vacuum distillation step in lube refining provides one or more raw stocks within the boiling range of about 550.degree. F. to 1050.degree. F., as well as the vacuum residuum byproduct. Often the vacuum charge is heated by a furnace means in order to vaporize a portion of the charge. The preheated charge normally enters a lower portion of the vacuum tower and the vapors therefrom rise through the tower where they are cooled in selected stages producing successively lighter liquids which are separately withdrawn as the sidestream raw stock products. In lube refining, excess liquid runback, known as overflash material, may be combined with the vacuum residuum and either withdrawn from the tower or charged to a deasphalting unit for further processing or dealt with in other conventional ways known to those skilled in the art. The overflash material may alternatively be withdrawn, recovered and charged directly to a solvent extraction unit. The presence of metallic impurities, asphaltenes and the like may render this material unsuitable for this step or likewise, for a catalytic processing step. Typical vacuum distillation systems are disclosed in U.S. Pat. Nos. 2,713,023, 3,886,062, 4,239,618 and 4,261,814, incorporated herein by reference in their entirety. Vacuum tower designs particularly germane to the present invention are disclosed in U.S. Pat. Nos. 3,929,626 and 3,989,616, which are hereby incorporated by reference in their entirety.
Following vacuum distillation, each raw stock is extracted with a solvent, e.g. furfural, phenol or chlorex, which is selective for aromatic hydrocarbons, removing undesirable components. The vacuum residuum usually requires an additional step, typically propane deasphalting, to remove asphaltic material prior to solvent extraction. The products produced for further processing into base stocks are known as raffinates. The raffinate from solvent refining is thereafter dewaxed by admixing with a solvent such as a blend of methyl ethyl ketone and toluene. for example and then processed into finished base stocks.
The solvent extraction step separates hydrocarbon mixtures into two phases; the previously described raffinate phase which contains substances of relatively high hydrogen to carbon ratio, often called paraffinic type materials, and an extract phase which contains substances of relatively low hydrogen to carbon ratio often called aromatic type materials. Solvent extraction is possible because different liquid compounds have different solutio affinities for each other and some combinations are completely miscible while other combinations are almost immiscible. The ability to distinguish between high carbon to hydrogen aromatic type and low carbon to hydrogen or paraffinic type materials is termed selectivity. The more finely this distinguishing can be done the higher the selectivity of the solvent.
Furfural is typical of a suitable solvent extraction agent. Its miscibility characteristics and physical properties permit use with both highly aromatic and highly paraffinic oils of wide boiling range. Diesel fuels and light and heavy lubricating stocks are refined with furfural. Furfural exhibits good selectivity at elevated temperatures (175.degree.-250.degree. F.). In a typical furfural solvent extraction unit for lubricating oils, the ray feed is introduced below or about at the center of the extraction tower. Furfural is fed into the top or upper portion of the tower. Recycled extract may be introduced into the lower section of the tower as reflux. Likewise, internal reflux is effected in the tower by the temperature gradient which is brought about by introducing the solvent at an elevated temperature and by intermediate cooling systems. Furfural solvent is recovered from the raffinate and extract phase streams or layers in suitable distillation and stripping equipment. The stripped and recovered solvent is then recycled.
While the furfural solvent extraction unit raffinate goes on to further processing, the extract from the operation often finds utility in a broad range of industrial applications. Applications for these aromatic extracts often vary according to the particular properties of the extracts, these properties largely a function of the feedstock used and unit conditions. For example, as described in "A New Look at Oils in Rubber" by H. F. Weindel and R. R. Terc, Rubber World, December, 1977, these extracts often find further utility as low and high viscosity aromatic extender oils for rubber processing. Bright stock extracts (BSE), obtained by solvent-refining deasphalted vacuum resides during the production of bright stocks, are also useful in rubber processing and find utility as ink oils as well. Like the lighter aromatic extracts, BSE's possess excellent solvent characteristics which lend themselves to great potential utility.
Besides having utility as a feedstock to the solvent extraction unit, the raffinate stream of the deasphalting unit can find further utility as a specialty oil. Depending on its characteristics, this stream, also known as deasphalted oil (DAO), can find utility as an extender oil for rubber processing, an ink oil, etc.
In recent years, concerns have arisen regarding the potential hazards associated with the use of various aromatic oils, DAO's and BSE's. As noted in U.S. Pat. No. 4,321,094, at col. 2, lines 9-14, " . . . many printing ink oils still contain proportions of aromatic hydrocarbons which either are proven to be carcinogenic, such as benzene, or are believed to be carcinogenic, such as toluene and polycyclic compounds. Clearly elimination of these from an ink would be desirable for health reasons." As a result of these concerns, many refiners are no longer willing to supply DAO's or aromatic extracts, including BSE's, for these specialty applications. Those refiners that continue to market these products must provide labels outlining the potential risks associated with the use of these products. This has led to the development and selection of alternate materials for applications previously fulfilled by DAO's and BSE's, as evidenced by U.S. Pat. Nos. 4,321,094 and 4,519,841. The use of these alternative solvents often carries with it the penalty of higher cost and inferior finished product quality.
To determine the relative carcinogenic activity of a deasphalted oil or an aromatic extract such as a BSE, a reliable test method for assaying such activity in complex hydrocarbon mixtures is required. A highly reproducible method showing strong correlation with the carcinogenic activity index of hydrocarbon mixtures is disclosed in U.S. Pat. No. 4,499,187, which is incorporated by reference in its entirety. From the testing of hydrocarbon samples as disclosed in U.S. Pat. No. 4,499,187, a property of the sample, known as its Mutagenicity Index (MI) is determined. Hydrocarbon mixtures exhibiting MI's less than or equal to 1.0 are known to be non-carcinogenic, while samples exhibiting MI's equal to about 0.0 are known to be completely free of mutagenic activity. It would be desirable to produce deasphalted oils and/or aromatic extracts such as bright stock extracts which are non-carcinogenic such that contact with same will not cause the development of cancerous growths in living tissue. It would be still more desirable to produce DAO's and/or BSE's which are free of mutagenic activity; that is, that contact with such products would not induce mutations in DNA and in living cells.
It is, therefore, an object of this invention to provide a substantially non-carcinogenic bright stock extract.
It is a further object of this invention to provide a substantially non-carcinogenic deasphalted oil.
It is another object of this invention to provide a process for making a substantially non-carcinogenic bright stock extract.
It is a yet further object of this invention to provide a process for making a substantially non-carcinogenic deasphalted oil.
Yet another object of this invention is to provide a bright stock extract substantially free from mutagenic activity.
It is still yet another object of this invention to provide a deasphalted oil substantially free from mutagenic activity.
Still yet a further object of this invention is to provide a process for making a bright stock extract which is substantially free from mutagenic activity.
It is still yet another object of this invention to provide a process for making a deasphalted oil which is substantially free from mutagenic activity.